There have been various investigations of lubricating compositions intended to improve the functioning of machines which are in contact in harsh environments of high speeds and large loads. For example, Japanese Laid-open Patent 2008-133440 proposes a lubricating composition which can be used in transmissions where increasing compactness has created conditions of running at high speeds and high loads. This lubricating composition incorporates, in base oils which are mineral oils and/or synthetic oils, metal dithiophosphates and poly(meth)acrylates which contain hydroxyl groups. Its anti-seizing performance is good, and it is possible to obtain a lubricating composition which has extreme-pressure properties the same as or better than with sulphur-phosphorus based additives, low fatigue characteristics, high oxidative stability and the prospect of longer life. A satisfactory lubricating composition can be obtained even under conditions where transmissions have been made more compact and are also running under high speeds and high loads.
However, lubrication mechanisms in rolling contact or rolling and sliding contact systems where a load (weight) is applied have aspects that are different from transmissions, and these mechanisms have been under investigation. For example, in Tribology Transactions, Vol. 53, 2010, page 658 it has been shown that a lubricating composition which forms an EHL (Elasto-Hydrodynamic Lubrication) oil film and so prevents interference between protuberances on sliding surfaces can be used as a lubricating composition for use in rolling contact or rolling-sliding contact systems such as roller bearings or gears, and especially as a lubricating composition for use in rolling contact or rolling-sliding contact systems under a load (weight).
According to Tribology Transactions, Vol. 53, 2010, page 658, the important elements in a lubricating composition which forms an EHL oil film are the minimum oil film thickness in line contact and the pressure-viscosity coefficient. The minimum oil film thickness is the minimum oil film thickness of the line contact gap, and so is the minimum thickness of the film of oil that is present in the line contact gap. It signifies the minimum condition for maintaining lubrication. The pressure-viscosity coefficient is a coefficient showing the relationship between the pressure applied in the contact system and the viscosity of the lubricating composition. It is the numerical value expressed by a in the Hamrock-Dowson formula, and the larger the value the higher the viscosity as the pressure increases. It shows a trend whereby a high oil film thickness is maintained under elasto-hydrodynamic lubricating conditions.
In Journal of Lubrication Technology, Transactions of ASME, 99 (April), 264 (1977) it is also disclosed that a lubricating composition which forms an EHL (elasto-hydrodynamic lubrication) oil film plays a role in preventing interference between protuberances on sliding surfaces in roller bearings, and the Hamrock-Dowson formula relating to point contact minimum oil thickness (Hmin: dimensionless minimum oil film thickness) and central oil film thickness (Hc: dimensionless central oil film thickness) is shown.
As a specific example of a lubricating composition which can be used in the bearings of high-speed main spindles having ceramic ball roller-bearings run in harsh environments of high speeds and large loads in the high-speed machining centres which process aeroplane parts and in particular metals such as titanium, there is the lubricating composition for use in ceramic lubrication proposed in Japanese Laid-open Patent 2008-179669. In this lubricating composition a base oil, being at least one kind of oil selected from mineral oils and/or synthetic oils, contains at least one kind of additive selected from the group consisting of acid amides obtained by reacting amines with saturated monocarboxylic acids of 12 to 30 carbons or unsaturated monocarboxylic acids of 18 to 24 carbons, sarcosinic acids, aspartic acid derivatives or succinic acid derivatives. If it is used even in the high-speed main spindles of machine tools which have ceramic ball roller-bearings run in harsh environments of high speeds and large loads, it displays satisfactory cooling properties and has good rust prevention, a high level of thermal and oxidative stability, and high extreme-pressure properties.